Metal materials and semiconductor materials have different work functions. At an interface between a metal material and a semiconductor material in a conventional thin film transistor, electrons flow from a material having a lower work function to another material having a higher work function, until two Femi levels of two materials equilibrate to a same level through the local exchange of carriers. At the equilibration point, either an ohmic contact or a Schottky contact is formed at the interface.
Schottky contact resistance limits frequency response of an apparatus. Charging and discharging of Schottky contact resistance result in additional energy consumption. In contrast, ohmic contact does not produce significant additional impedance, and does not change the balanced carrier concentration in the active layer in any significant manner. Thus, the ohmic contact is the ideal contact type in a thin film transistor.